Radio frequency (RF) and microwave power amplifiers are used in the field of communication as they generate relatively high amounts of power that is useful in wireless communication systems. These RF and microwave power amplifiers are biased with various types of circuits. A well known type of circuit used for biasing is a current mirror circuit. In a typical current mirror circuit, an output current follows, or mirrors, the input current, or is a proportionate multiple of the input current. However, inherent problems such as changes in supply voltage, ambient temperature variations, and manufacturing variations prevent the output current from accurately mirroring the input current. Furthermore, in many RF and Microwave power amplifiers the bias point of the transistors shift as the supply voltage varies due to the “Early voltage effect”.
The Early voltage effect is a variation in an effective width of the base region of the transistor due to a variation in the supply voltage (the collector voltage) across the base-to-collector terminal of the transistor. When biased, the effective width of the base region is less than the actual width of the base region in the transistor due to the presence of depletion regions at one or more of the emitter-base junction and the base-collector junction. Thus, when the collector voltage increases the area of the depletion region also increases resulting in increased current gain. In the current mirror circuit, current ratios across mirroring transistors are based on areas of the mirroring transistors. However, due to the Early voltage effect, with an increase in the supply voltage at the collector terminals, the current across the collector terminals of the mirroring transistors increases. This increase causes errors in the current ratios across the mirroring transistors in the current mirror circuit. Although current mirror circuits are a standard part of most analog integrated circuits, the standard current mirror circuit configurations do not address the need for collector supply voltage compensation of RF power amplifiers.
In view of the foregoing, a compensating current mirror circuit that is adaptive to the Early voltage effect, variations in supply voltage, ambient temperature changes, and tolerant to manufacturing variations is desirable.